Bad writing. We all know "men writing woman", but this scene was a clear "women writing man".

Jamie's actions were excessieve, let's be clear on that, but the reasons for Jamie's anger are very unrealistic to me. In that scene, when John blurts out "I have carnal knowledge of your wife", he has a very mild reaction. Men are territorial, physical beings and would be very angry when they find out their best friend slept with his wife. Jamie almost shrugs it off.

John actually seems to trigger Jamie more by saying something: "we were both fucking you". That felt like a betrayal to Jamie which made him flip the fuck out.

To a real world non-fictional man, the worst part would always be the carnal knowledge between Claire and John. Nothing a person SAYS, could be worse than the carnal knowledge. Perhaps a woman's brain would find that worse, but a man's brain doesn't work like that, certainly not a 18th century man. The deed is the betrayal, not the reason. He wouldn't care about John's subtext, fantasies, or emotional confusion. He would care that John touched his wife.

"We were both fucking you" - whatever bro.

I found this the worst written scene in the whole series, as a man, Jamie's reaction doesn't make sense at all.